The present invention relates generally to well servicing operations, and, more particularly, to devices, systems and methods useful in blending fluids, mixtures, and/or slurries used in well servicing operations.
Well treatments often performed in the oil industry requiring mixing or blending of dry particulate material with a liquid or gel. Such blended materials are used in various well treatment and completion procedures. For example, well treatment fluids are utilized in fracturing formations, to increase or control hydrostatic pressure, etc. Proppant, sand, and other dry powder solids are blended with a liquid, liquid mixture or gel, to create a blended liquid having particulate entrained in the liquid. Blending to essentially homogeneous uniformity is a problem in the oil service industry, particularly for high particulate concentrations, at high blending rates, and for more viscous fluids, such as gels. Such blended fluids are typically made using a dry particulate mixed with a liquid, often water but also hydrocarbon-based and other fluids. Such blending procedures have inherent problems, particularly at remote sites or when large volumes are required. Other problems typically encountered are air entrainment in the fluid, inadequate solids wetting, and dispersion of solids. Various mixing methods have been tried with varying degrees of success.
Conventional blenders have been either the open-top tub blenders or centrifugal blenders. Open-top tub blenders and their associated short-comings, limitations and problems are discussed in U.S. Pat. No. 7,353,875 to Stephenson, issued Apr. 8, 2008, which is incorporated herein by reference for all purposes. Consequently, it is often desirable to use a centrifugal blender system.
Generally, there are three types of centrifugal blender system in use. The Condor-type blender uses an integral impeller design having a common base with both upper and lower vanes. The lower impeller vanes pump fluid into the volute chamber. The upper expeller vanes expel the sand into the volute. The suction and discharge functions are provide by a common shaft and impeller. The common shaft and impeller arrangement requires compromise in the design of the impeller and also requires sand injection occur at relatively higher discharge pressures (e.g., 60 psi or more), causing high erosion and air entrainment. Condor-type mixers are available from Condor Engineering and Manufacturing, LLC. The Crown-type blender utilizes two separate impeller-type devices driven by independent motors. A conventional suction pump having an impeller supplies fluid at required discharge pressure (e.g., 60 psi or more) to a mixing impeller, where the sand is injected into the fluid stream. The sand injection process is forced to occur at the discharge pressure (e.g., 60 psi or more), which translates to high wear and air entrainment. The three independent impeller type blender (see U.S. Pat. No. 7,353,875 to Stephenson, et al.) utilizes a suction impeller pump to supply low pressure fluid to the mixer (e.g., 10-15 psi) where the mixer expeller injects the sand into the low pressure stream. This requires relatively lower expeller speeds and thus results in lower erosion rates and reduced air entrainment. The slurry is then boosted to discharge pressure (e.g., 60 psi or more) by a third impeller in a discharge or slurry pump.
For further disclosure regarding use and structure of these blender types see U.S. Pat. No. 4,453,829 to Althouse, III; U.S. Pat. No. 4,614,435 to McIntire; U.S. Pat. No. 4,671,665 to McIntire; U.S. Pat. No. 4,808,004 to McIntire et al.; U.S. Pat. No. 4,239,396 to Arribau et al.; U.S. Pat. No. 4,460,276 to Arribau et al.; U.S. Pat. No. 4,850,702 to Arribau et al.; U.S. Pat. No. 4,915,505 to Arribau et al.; U.S. Pat. No. 6,193,402 to Grimland et al.; U.S. Pat. No. 7,334,937 to Arribau; U.S. Pat. No. 7,353,875 to Stephenson, et al.; U.S. Pat. No. 7,048,432 to Phillippi, et al., each of which is hereby incorporated herein in its entirety for all purposes.
Separating the suction pumping and/or discharge pumping from the blending process by utilizing dedicated pumps has led to advances in the art. However, problems remain with the expeller used in the blending step. Existing closed blending systems used in oil field operations consist of either large, deep impellers with vanes adapted from centrifugal pump applications, such as a “Crown” blender, now believed to be commercially available from Stewart and Stevenson as pressurized mixing chamber blenders, or specialized expeller-and-impeller designs with complicated dual mode, clean side/dirty side systems which accomplish pressure building and particulate mixing function. These designs are focused on the process of mixing proppant into a pressurized fluid container with and without an external suction pump. Other remaining problems include conveying proppant at sufficient rates, reducing introduction of air into the fluid from the action of the expeller vanes, minimizing torque requirements to spin the expeller, preventing backflow of treatment fluid into the expeller eye, and being relatively insensitive to the inlet velocity of the proppant. Consequently, there is a need for improved blending apparatus and expeller design.